1. Field of the Invention
The present invention relates to a method and system for processing RI images in a SPECT apparatus.
2. Description of the Related Art
In a conventional SPECT (single photon emission computed tomography) apparatus shown in FIG. 1, when RI (radioisotope) is injected to a subject to be examined (not shown), radiation from the subject is detected by a detector 1. Position data X and Y and unblank data UNB associated with the radiation detected by the detector 1 are stored in a memory 3 through an A/D converter 2. During detection of radiation, since the detector 1 is rotated about the axis of the subject by a detector rotating unit 4, data detected by the detector 1 at various rotational positions are stored in the memory 3. A rotational position signal representing the rotational position of the detector 1 is output from the detector rotating unit 4 to a memory controller 5. The memory controller 5 causes the memory 3 to output the data stored therein to an image reconstructing unit 6 in response to the rotational position signal. An RI image (SPECT image) is reconstructed by the image reconstructing unit 6 on the basis of the data at each rotational position of the detector 1, and the reconstructed image is displayed on a display unit 8 through a display memory 7.
In the above-mentioned SPECT apparatus, when the subject is placed on a berth so that the axis of the subject substantially coincides with the central axis of rotation of the detector, an RI image 10 to be acquired corresponds to a plane perpendicular to an axis of body 9, as shown in FIG. 2.
As the performance of the SPECT apparatus has been improved in recent years, a case wherein an RI image of a head is acquired for the purpose of diagnosis of the head, e.g., an examination of cerebovascular disease, has, been increased. When RI images are acquired in diagnosis, a head photographing standard line 30 shown in FIG. 3, i.e., a line (to be referred on an OM line hereinafter) which connects an eye to an ear hole of the subject, must be referred to, as in a case wherein an X-ray CT image or an MR (magnetic resonance) image is acquired. In the conventional SPECT apparatus, therefore, the posture of the subject is set so that an OM line is perpendicular to the center of rotation of the detector in order to acquire RI images (slice images) on the OM line.
It is, however, often difficult to set such a posture of a patient which has a trouble in his or her backbone, neck, or the like. In this case, RI images on the OM line cannot be acquired. In order to solve this problem, a SPECT apparatus which can acquire RI images without changing the posture of a patient has been developed. In this SPECT apparatus, the gradient of the OM line is measured in advance by an operator, and the gradient of the OM line is off-line input during image reconstruction processing. According to this apparatus, it is determined that the gradient of an image is equal to that of the OM line. However, it is difficult to specify a tomogram corresponding to the OM line among a plurality of tomograms.
Thus, a demand has arisen for developing a SPECT apparatus which can acquire RI images on a photographing standard line set in a subject regardless of a posture of the subject.